1. Field of the Invention
This invention generally relates to suction piles for use with bottom-founded offshore structures resting in relatively shallow waters in which environmental conditions can present severe lateral load threats sufficient to displace and shift the structures from their normal rest positions.
2. Description of the Prior Art
Bottom-founded, mobile or stationary, submersible structures are required in various types of offshore operations, including scientific surveys and oil and gas drilling and production facilities.
The known relevant prior art includes bottom-founded offshore structures and, particularly, but not exclusively, mobile or stationary submersible platforms for carrying out oil and gas drilling and production operations throughout the world but primarily within the continental shelf of the Gulf of Mexico, especially in its Louisiana zone, which continues to be under intense investigation for its potential oil and gas resources. These types of water zones are of great concern to operators of bottom-founded mobile or stationary submersible structures, because they are prone to experience environmental lateral load threats that can create formidable obstacles to achieving uninterrupted use of fixed bottom-founded, drilling and production offshore installations.
Under normal operating conditions, the horizontal or lateral loads, generated by strong winds, heavy seas, wave surges, subsea currents, and shifting soft soil layers, are generally insufficient to cause a structure to slide from its rest position, because the natural shear forces between the foundation layers, and the frictional forces at the interface between the structure bottom and the seabed, combine to provide to the structure adequate resistance against its bottom sliding over the seabed within its seaway location.
However, under severe and often unexpected operating conditions, the combined shear and frictional forces can be over-powered, as during the hurricane season or other such environmental events, to cause the structure to slide or shift from its rest position on the seabed. This dreaded structure sliding or shifting phenomenon is known in this insular art as the "bottom sliding problem".
Of course, any sliding, even over a minor distance, can disrupt the vertical alignment between the structure and the oil or gas well under it, and produce potential catastrophic consequences from damaged wellheads, associated equipments, ruptured pipes, hydrocarbon spillovers into the sea, etc.
It is important to note that, because environmental and operational conditions are distinct within shallow warm waters, shallow ice waters, and deep water regions, the respective platform arts have become separate and distinct, as is well known to those skilled in these arts. Even in large, integrated oil corporations, workers in these distinct water regions operate within different corporate divisions, which frequently are under separate managements.
Over the years, operators in the shallow waters of the Gulf of Mexico have made many attempts to find a reliable, dependable, and economical technical solution to this bottom sliding problem.
A "slender-pile" approach to solving the bottom sliding problem involved driving into the sea bottom, through hull-attached guides, long slender piles having length-to-diameter ratios on the order of 30:1. Typically, these slender piles were 16, 20 or 30 inches in diameter and more than 50 feet long. Such slender piles frequently failed to supply enough additional sliding resistance to prevent the submersible structure from sliding in response to mild-to-moderate environmental loads. In addition to being costly, unreliable, time consuming to imbed and to extract, these slender piles proved to be uneconomical and operationally disadvantageous to potentially reduce the bottom sliding problem. Using larger diameter piles in sufficient numbers could have increased the structure's sliding resistance, but such piles would have been too heavy, expensive and time-consuming for the structure's cranes to lift, install, and extract.
A "skirt" approach to solving the bottom sliding problem, which is still being used, relies on adding a bottom skirt to an offshore structure so that it will self-penetrate into soft seabeds and thus hopefully increase the platform's frictional resistance capacity at the interface with the seabed. But such a skirt can hardly be expected to penetrate into dense or clay formations. Even in soft seabeds, the skirt's frictional resistance capacity increase is at best unreliable, erratic and unpredictable.
A "gravity" approach, which can be combined with the "skirt" approach, relies on adding extra weight at least to the bottom section of the structure, as by using concrete, in full or in part, as the building material for the walls and floors of the structure. This gravity approach can increase substantially the cost of building the structure and, in any event, does not altogether eliminate the bottom sliding problem.
A more recent approach, for use in the arctic shallow water regions of Canada and Alaska, is described in U.S. Pat. No. 4,579,481. A concrete-steel drilling platform 10 uses dozens of spud piles 42 within the peripheral walls of its substructure 12. Piles 42 are designed for use in about fifty feet mean arctic water depths. Each pile 42 has a 7' diameter and a 110' length, yielding a length-to-diameter ratio of about 16:1. A highly complex mechanical bushing 60, between each pile and the platform structure, is used for load transfer, in a manner as to allow the misalignment of pile 42 via pivoting of the bushing. Under over-load conditions, and prior to inflicting damage to the platform itself, spud piles 42 are permitted to flex between vertically-spaced-apart fulcrum points, which is to be expected in view of their relatively high 16:1 length-to-diameter ratio. Each pile 42 is hung from a deck crane with its top end being above water to allow a pile driver or vibratory hammer to imbed the pile into the foundation beneath the seabed.
Typically, the cranes on such an oil-producing structure are of insufficient size and power to handle very long and heavy spud piles. Using larger cranes is not practical because they would occupy precious deck space needed for carrying cargo, and also would interfere with normal deck operations. To avoid such problems, large cranes and pile drivers on auxiliary vessels are employed for pile imbedment and extraction.
But, such external equipments and services may not be available on short notice, especially under abnormal operating conditions, generated by strong winds, heavy seas, wave surges, subsea currents, storms or the like. Therefore, it is frequently more convenient to just sever the already installed piles, abandon them in the ground, and treat them as expendable albeit costly commodities.